It is well known in the art to apply anti-skid control systems to vehicles to modify the behavior of the wheels of a motor vehicle during a braking operation. Anti-skid systems are typically designed to sequentially increase and reduce brake fluid pressure in a desirable manner to prevent the wheels from becoming locked. Known anti-skid braking control systems use various mathematical manipulations to determine the speed of a vehicle for comparison with the rotational speed of a given vehicle wheel to calculate wheel “slip” for that wheel, with wheel slip being a parameter needed in order to make decisions concerning wheel lock control.
Additionally, it is known to provide traction control systems that attempt to maximize vehicle acceleration and prevent overspin of the wheels. Many all wheel drive vehicles have drivelines with actively controlled torque transfer devices. In the case of a so called “on-demand” four wheel drive system, a powertrain delivers torque to a first set of wheels, which are considered primary drive wheels, at all times while torque sent to a second set of wheels, which are considered secondary drive wheels, only under certain conditions. For example, when the traction control system senses that traction has become limited at the first set of wheels, the torque transfer device is directed to send torque to the second set of wheels.
The prior art anti-skid and traction control systems, which are reactive in nature, are generally unsatisfactory. These systems allow the primary drive wheels to slip and then, only after the slip has been detected, the traction control system will activate the torque transfer devices to limit the slip. Of course, with such a system, a driver will sense the slipping wheels, along with a shock as the torque transfer device engages. The situation is aggravated when the vehicle is on a hill. When accelerating on a hill, the vehicle's pitch may be such that weight is transferred from the primary drive wheels, causing them to slip. The resulting momentary slip will cause driver discomfort and, even worse, could lead to vehicle instability.
Since the surface condition of a road and the slope or grade of the road surface directly affects braking and traction, some prior art control devices, such as braking control devices and traction control devices, are designed to determine a road surface coefficient of friction and a hill slope. For example, U.S. Pat. No. 5,132,906 discloses a method of estimating road surface friction and hill slope. The hill slope and road surface coefficient of friction are estimated by processing signals representing different characteristics of the vehicle. For example, with respect to each wheel, signals representing radius, speed, change in speed, rotating inertia and torque value are processed. In addition, signals related to a vehicle mass, velocity and change in velocity of the overall vehicle are processed. The calculated hill slope and road surface coefficient of friction are used to control traction as the vehicle climbs a slippery hill. Similarly, U.S. Patent Application Publication No. 2007/0129871 discloses a feed forward distribution of torque in a vehicle based on an estimated hill grade and a coefficient of friction of a road surface. The estimate of hill slope is made using wheel sensors and longitudinal acceleration sensors. Therefore, in each case, the hill slope is obtained from the output of speed sensors. The problem with using wheel speed sensors is that, on low traction surfaces, wheels tend to slip and, once they are slipping, the wheel speed sensor outputs will become erratic. With this arrangement, the torque controller will not be able to control the torque distribution in a reliable manner.
As can be seen by the above discussion, there is a need in the art for a system that will effectively distribute torque among vehicle wheels to prevent wheel slip as a vehicle travels over a slippery hill. More particularly, there is a need for such a system that does not rely on wheel speed sensors and is able to proactively prevent wheels from slipping, rather than merely reacting to wheel slippage.